The present invention relates to a document scanning system, and more particularly, to a system for scanning a flat document at a constant scan rate across the document surface using a rotating mirror.
Copiers, duplicators, and other types of document processing devices require an arrangement for optically scanning an image from an original document and directing the light reflected from the image to an image processing arrangement, such as a bank of photosensitive electrical transducers, or a photoconductive drum as is commonly utilized in xerographic duplication. One technique which has been used to scan a flat document is to place the document on a moving support and transport the document past a scan station. Fixed scanning optics adjacent the scan station then scan the moving document. Such an arrangement limits the scanning rate, however, since this rate is dependent upon the rate of movement of the document past the scanning station. Additionally, the document transport may tend to be somewhat bulky.
Another approach which has in the past been commonly used is to place the document on a document supporting transparent platen and to effect scanning of the document by means of a rotating mirror which forms a part of an otherwise stationary optical scanning arrangement. It will be appreciated that if the document is held flat and, further, if the mirror is rotated at a constant angular velocity, the scanning velocity along the document will be non-uniform. In order to eliminate this non-uniformity, it has been common for scanning arrangements to use a curved transparent platen for supporting the document, with the rotating mirror positioned substantially at the center of curvature of the platen. With such an arrangement, a uniform rate of rotation of the mirror results in a uniform scanning rate across the document.
A curved document supporting platen may be somewhat undesirable, however, for a number of reasons. First, it may be necessary to scan a document which is printed upon somewhat inflexible material such that the document may not readily be deformed into the curved shape of the platen. Books having a rigid binding, for instance, may not be readily flexed. Additionally, it may be desired to scan a document which is conveyed to and from the scanning arrangement by a conveyer, such as a belt. The belt provides a substantially flat document support, and it is desirable to scan the document without removing the document from the belt.
One approach to scanning a document positioned on a flat document support is disclosed in U.S. Pat. No. 3,814,494, issued June 4, 1974, to Shiraghi et al. The device disclosed in the Shiraghi et al patent utilizes a relatively complicated camming arrangement to rotate a mirror in the scanning optics at a non-uniform angular velocity. The velocity of rotation of the mirror is varied appropriately to produce a uniform scan rate across the document. The Shiraghi et al scanning device is only as accurate, however, as the camming arrangement.
U.S. Pat. No. 3,389,403, issued June 18, 1968, to Cottingham et al, discloses a system in which a rotating mirror causes a laser beam to sweep across a flat strip of film or other light-sensitive material. A second parallel laser beam is directed by the rotating mirror to a timing bar having evenly spaced slots for receiving the beam and directing it to a photo-diode at the end of the bar. The mirror is rotated at a uniform angular velocity and thus the scanning rate across both the film and the timing bar is non-uniform. The output pulses from the photo-diode, therefore, are also non-uniform in frequency. These pulses are used to time the application of data to the laser, thus modifying the data flow sufficiently such that successive bits of data control operation of the laser as the beam strikes successive, evenly spaced scan points on the film.
U.S. Pat. No. 4,041,454, issued Aug. 9, 1977, to Shepard et al discloses scanning a document by reflecting a beam of light to the document with a rotating mirror. The beam is also split by a half slivered mirror and directed to a position reference stip having a plurality of evenly spaced markings along its entire extent. Light reflected from the document is directed to a first photo-sensitive transducer, while light reflected from the reference strip is directed to a second photo-sensitive transducer. The Shepard et al system scans across the document at a non-uniform scanning rate and utilizes the output pulses from the photo-transducer associated with the position reference strip to control the sampling rate of the output from the scanning transducer. This produces samples indicative of the image density of the scanned document at uniformly positioned scan points across the document, even though the document is scanned at a non-uniform rate. A similar system is shown in U.S. Pat. No. 4,037,231, issued July 19, 1977, to Broyles et al. In the Broyles device, a mirror driven by a resonant mechanical oscillator is used to scan a modulated laser beam across a rotating dielectric drum to form a charge pattern on the drum which is ultimately used in a printing process. The scan velocity of the laser beam across the drum varies sinusoidally because of the resonant nature of the drive system. The data flow which is used to modulate the laser beam is rate controlled by a variable clock to accomodate the periodic variations in scanning velocity.
U.S. Pat. No. 3,997,828, issued Dec. 14, 1976, to Bottcher et al and U.S. Pat. No. 4,060,907, issued Dec. 6, 1977, to Van Hook, both disclose scanning of documents or images on rotating drum supports in which reference marks or slots on the drum support or associated equipment provide, when scanned, reference pulses defining the position of the drum. These reference pulses are used to control the drum rotation.
Accordingly, it is seen that there is a need for a simple, reliable system for scanning a stationary, flat document at a uniform scan rate.